Integrated circuits are well-known for the limited amount of circuit area that they require. This is particularly true of filters made of integrated circuits. However, active filters conventionally require the use of a negative resistance (-R) component that inherently makes them somewhat temperature sensitive and unstable. This -R component is generally used to produce inductance terms in the transfer function. Further, inductor coils use significantly more circuit area than do resistors or capacitors. It is therefore desirable to have an active filter that does not require the use of inductor coils.
The use of active elements in filters has been known for many years. For instance, Adams et al. in "Active Filters for UHF and Microwave Frequencies", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-17, No. 9, Sept. 1969, pages 662-670 describes some basic single-transistor designs using inductors. Hara et al. in "Broad Band Monolithic Microwave Active Inductor and Application to a Miniaturized Wide Band Amplifier", IEEE 1988 Microwave and Millimeter-Wave Monolithic Circuits Symposium, pages 117-120 disclose the use of a cascode transistor circuit configured to function as an active inductor. However, an active matching amplifier they describe that uses the active inductor also uses inductor coils, based on the negative resistance principle.
It has been found that a cascode circuit can be configured to function as a "super transistor". A cascode circuit typically includes a first input transistor, such as a FET having a common source, coupled in series with a second output transistor, such as a FET having a common gate. In "A High-Swing, High-Impedance MOS Cascode Circuit", IEEE Journal of Solid-State Circuits, Vol. 25, No. 1, Feb. 1990, pages 289-297, Sackinger et al. describe connecting a third, regulating transistor between the gate and source of the second transistor and ground. The authors refer to this configuration as a regulated cascode circuit. The output impedance of this circuit is higher and the usable output-voltage range is enlarged compared to those of a simple cascode circuit. This circuit has a fixed configuration and so does not accommodate various filtering requirements.
An active filter that does not require, but allows the use of inductors and which is usable in a radio-frequency signal path is therefore desirable.